This invention relates generally to grounding and indicating devices. More particularly, the invention relates to an intrinsically safe indicator and grounding device for use with tank vehicles, drums or other portable containers during the transfer and storage of combustible materials.
The transfer and storage of combustible materials requires careful handling to minimize the possibility of explosions. A static discharge from the vehicle or container could trigger such an explosion, causing severe damage. The possibility of a static build-up increases whenever the combustible material is placed in or removed from a container. Circulation of the combustible material causes a potentially dangerous static charge build-up.
Thus, it has become customary to ground containers when combustible material is being transferred, and to provide an indication to an operator that he or she has properly grounded the container. Some representative examples of such a grounding and indicating device may be found in U.S. Pat. Nos. 3,290,668, 3,343,154, 3,996,496, 4,225,899 and 4,642,554. The device shown in U.S. Pat. No. 3,290,668, issued to Perretta, and assigned to the assignee of the present invention, has been the industry standard for over twenty years.
U.S. Pat. No. 3,290,668, the teaching of which is incorporated by reference herein, discloses a grounding clip for attaching to the container to insure that the container is grounded. When the grounding clip connects to a container, it closes the associated circuit, causing a miniature lamp to ignite. Ignition of the lamp triggers a photo-conductive cell, which, in turn, energizes a sensitive relay that is normally-open. The sensitive relay closes, thereby energizing a load relay and causing it to switch. When energized, the load relay connects a green lamp to the power supply and disconnects a red lamp from the power supply. A valve is connected in parallel across the green lamp, which is energized together with the green lamp. Once energized, the valve permits the combustible material to be pumped through a pump interlock.
One of the key requirements of the grounding circuit in U.S. Pat. No. 3,290,668 is that the portion of the circuit connected to the grounding clip must be operated at a relatively low voltage, so that even if there is a failure in this portion of the circuit, the discharge from such a failure will be so small that the combustible material will not be ignited or affected in any way. As a result, the circuit elements must be capable of low power operation. This places certain limitations on the circuit elements that can be used.
Yet another requirement is that any circuit failure should be such that it prevents the pumping of combustible material until the failure has been repaired. The grounding device, therefore, preferably should leave the red lamp on when it fails. If the device fails and leaves on the green lamp, a dangerous situation arises.
Still another problem with the prior art grounding devices is the requirement of a photo-conductive cell to isolate the intrinsically safe circuit from the rest of the circuitry. These cells are extremely susceptible to ambient light, and thus they must be carefully shielded to prevent erroneous triggering. Those skilled in the art have realized that it would be advantageous to use a solid state optoisolator or optoelectronic coupler, which are basically immune from the effects of ambient light, in place of the lamp and photocell. The problem, however, is that such an optoisolator cannot be used with a relay, because a relay constitutes an inductive load, which causes a phase shift, with the voltage leading the current by 90.degree.. This causes a major problem because, as the current through the output of the optoisolator goes to zero, the voltage across the relay, and thus across the output of the optoisolator, is at a maximum. This results in a voltage spike across the output terminals of the optoisolator, which will cause the optoisolator to fail prematurely, usually within three or four applications.
To date, no one skilled in the art has been able to develop a grounding device that uses an optoisolator that works effectively with a relay, without burning out in an unreasonably short time period. Some designers have attempted to utilize triacs in the circuit as a substitute for relays. The problem with using triacs is that triacs tend to fail by shorting out, as compared to a relay, which normally fails in an open state. A shorted-out triac can cause the green lamp to remain lit even when the container is not grounded.
Another problem with prior art grounding devices is that these devices are designed on the assumption that the intrinsically safe circuit is, in fact, grounded. If the intrinsically safe circuit is not grounded, then the clip will not ground the container. Such a situation can occur, for example, when an operator or field installer fails to properly connect the grounding device to ground.
These and other problems with the prior art grounding and indicating devices have necessitated the development of the present invention.